1. Field of the Invention
The invention relates generally to semiconductor microelectromechanical devices or micromechanical force sensors that can be used to detect small forces or flexures generated from chemo-mechanical stress, thermal stress, electromagnetic fields, and the like. More particularly, but not limited to, the invention relates to integrated piezoresistive accelerometers and pressure sensors that may be manufactured on a single chip.
2. Description of the Related Art
Advances in semiconductor microelectronic sensors have served to greatly reduce the size and cost of such sensors. The electrical and mechanical properties of silicon microsensors have been well chronicled. For example, refer to Kurt E. Petersen, “Silicon as a Mechanical Material,” Proceedings of the IEEE, vol. 70, No. 5, May 1982. Moreover, there is a large and growing body of knowledge concerning techniques for constructing silicon microstructures, commonly referred to as “micromachining.” See, for example, Bryzek, Petersen and McCulley, “Micromachines on the March,” IEEE Spectrum, May 1994, pp. 20-31.
Thus, silicon micromachining and semiconductor microelectronic sensors have blossomed into a vital industry with numerous practical applications. For instance, micromachined silicon pressure sensors, acceleration sensors, flow sensor, and the like have found their way into various applications and industries ranging from medical instruments to automobiles. The high strength, elasticity, and resilience of silicon makes it an ideal base material for resonant structures that may, for example, be useful for electronic frequency control or sensor structures. Even consumer items such as watches, scuba diving equipment, hand-held tire pressure gages, and inflatable tennis shoes may soon incorporate silicon micromachined sensors.
The demand for silicon sensors in ever expanding fields of use continues to fuel a need for new and different silicon microsensor geometries and configurations optimized for particular environments and applications. Unfortunately, a drawback of traditional bulk silicon micromachining techniques has been that the contours and geometries of the resulting silicon microstructures have been significantly limited by these fabrication methods. For instance, etching silicon structures with conventional etching techniques is constrained, in part, by the crystal orientations of silicon substrates, which limits the geometry and miniaturization efforts of many desired structures.
The increasing use of microsensors to measure pressure or acceleration has spurred the development of small silicon plate structures used, for example, as capacitors and to produce electrostatic forces. For instance, there exist microsensors that measure capacitance using an array of interdigitated polysilicon plates. Similarly, there exist microsensors that produce electrostatic forces using an array of interdigitated plates. Further, there exist microsensors that measure the flexure, or bending, of silicon structures in response to forces such as weight or acceleration.
In many applications it is desired to obtain both pressure and acceleration measurements. In such applications, fabricating both pressure and accelerometer sensors on a single chip would be advantageous. As pressure and accelerometer sensors are fabricated to smaller dimensions, it is desired to integrate both types of sensors on a single chip and at the same time optimize the material and structural characteristics as well as the methods of manufacturing. It is further desired to manufacture small accelerometer sensors with high sensitivity. Consequently, there has been a need for a single chip integrated accelerometer sensors and pressure sensors, and a method of fabricating accelerometer sensors and pressure sensors on a single chip.
The expanding fields of use of micromechanical devices in general, and of accelerometers and pressure sensors in particular, has created a demand for even smaller devices. Unfortunately, there has been difficulty producing smaller devices that are also highly sensitive to small changes in acceleration or pressure. For example, there has been a need for a smaller accelerometer that combines sufficiently thin flexure structures with a sufficiently large proof mass (or seismic mass) to be responsive to small changes in acceleration.
The present invention meets these needs.